1. Field of the Invention
the present invention is directed to aqueously dispersed, heat-curable coating compositions containing water dispersible blocked polyisocyanates, water dispersible hydroxyl-terminated prepolymers and epoxy resins, and to coatings prepared from these compositions.
2. Description of the Prior Art
Mixtures of blocked polyisocyanates and coreactants such as polyhydroxyl compounds are known in the art. For example, U.S. Pat. No. 4,098,933 is directed to the preparation of water dispersible blocked polyisocyanates and to their use as crosslinking agents for other water dispersible coreactants such as polyhydroxyl compounds. U.S. Pat. No. 4,284,544 is directed to compositions containing water dispersible blocked polyisocyanates and polyhydroxyl coreactants which are not dispersible in water. Finally, U.S. Pat. No. 4,608,413 is directed to mixtures of water dispersible blocked polyisocyanate prepolymers and water dispersible polyhydroxyl prepolymers.
It has not been possible to prepare coatings from the above-described aqueous compositions which have good solvent resistant and high hardness values, and yet maintain good impact strength, flexibility and hydrolytic stability. To achieve good solvent resistance and high hardness values, it is necessary for the coatings to have a high crosslink density; however, to achieve the necessary crosslink density, the impact strength and flexibility generally decrease. In order to provide water dispersible blocked polyisocyanate prepolymers and water dispersible hydroxyl-terminated prepolymers suitable for preparing coatings with high crosslink densities, it is necessary to incorporate relatively large amounts of internal emulsifiers which results in coatings with reduced hydrolytic stability.
Accordingly, it is an object of the present invention to provide storage stable aqueous compositions which can be used to prepare coatings having excellent solvent resistance and high hardness as well as good impact strength, flexibility and hydrolytic stability. This object may be achieved according to the present invention which is described hereinafter.